Recent automatic transmissions have been demanded to be light and small and sought to be improved in power transmission capability in connection with the increased power output of the engines with which the transmissions are used in combination. There are some automatic transmissions or continuously variable transmissions that control the lock-up clutch built in the torque converter to be slipped at a low velocity (slip lock-up control). These transmissions have been improved with the slip lock-up control so that the drive feeling can be improved by absorbing a torque variation and the engine torque can be transmitted to the transmission mechanism efficiently. Some of the continuously variable transmissions are provided with a wet starting clutch which is initially allowed to slip and then coupled so as to start the vehicle smoothly from the halt, that is a so-called slip control. Lubricating oils used for the transmissions wherein slip control for the lock-up clutch or starting clutch is carried out are required to provide excellent torque capacity and less shifting shock as well as to have excellent and long-lasting initial anti-shudder properties.
There have been proposed transmission fluids, in which a friction modifier, a metallic detergent, an ashless dispersant, and an anti-wear agent are optimally added so as to retain the friction characteristics of a lock-up clutch in a good condition and provide long-lasting initial anti-shudder properties (see Patent Documents 1 to 7 below).
For example, Patent Document 1 discloses a transmission lubricating oil composition comprising a specific calcium salicylate, an SP-based extreme pressure additive, a specific succinimide and a boron-containing ashless dispersant, each in a specific amount, which composition exhibits excellent properties such as excellent anti-shudder properties and long-lasting fatigue life. Patent Document 2 discloses a continuously variable transmission lubricating oil composition containing an organic acid metal salt with a specific structure, an anti-wear agent, and a boron-containing succinimide, as essential components, which composition has both higher friction coefficient between metals and anti-shudder properties for a slip control mechanism. Patent Document 3 discloses a long-lasting continuously variable transmission lubricating oil composition comprising calcium salicylate, a phosphorous-containing anti-wear agent, a friction modifier, and a dispersant type viscosity index improver, which composition has both a higher friction coefficient between metals and anti-shudder properties for a slip control mechanism. Patent Document 4 discloses a lubricating oil composition comprising a dithiocarbamate compound, a condensate of a branched fatty acid having 8 to 30 carbon atoms and amine, and an aminic anti-oxidant, which composition has excellent and long-lasting anti-shudder properties. Patent Document 5 discloses an automatic transmission fluid composition comprising calcium sulfonate, phosphorous acid esters and further a sarcosine derivative or a reaction product of a carboxylic acid and amine, which composition has long-lasting anti-shudder properties for a slip lock-up mechanism and long-lasting properties to prevent scratch noise in a belt type continuously variable transmission. Patent Document 6 discloses an automatic transmission fluid composition comprising a specific alkaline earth metal sulfonate in a specific amount, which composition is excellent in oxidation stability as a fluid used for an automatic transmission with a slip control mechanism and has long-lasting anti-shudder properties. Patent Document 7 discloses an automatic transmission fluid comprising calcium salicylate, magnesium salicylate, a specific amount of a friction modifier and a specific amount of a boric acid-modified succinimide, with excellent anti-shudder properties and a certain torque capacity.
However, according to studies conducted by the inventors of the present invention, it was found that the use of a salicylate detergent as a metallic detergent made it difficult to retain sufficient torque capacity at high temperatures. It was also found that when a sulfonate detergent was used, the resulting composition was still insufficient in anti-shudder properties and often failed to retain sufficient torque capacity. On the other hand, it was found that an enhancement in torque capacity caused wet-type friction materials to wear severely and that the long use of the resulting composition made it difficult to retain initial torque capacity and shifting characteristics sufficiently and anti-shudder durability tended to be poor. For wet-type friction materials containing carbonaceous materials, which is excellent in heat resistance, such as carbon fiber, graphite, and carbon black, there is concern that conventional transmission fluids can not exhibit the above-described performances and in particular would be poor in anti-shudder properties due to the adsorptivity of various additives to the materials.
Therefore, there is a need for a lubricating oil composition with high torque capacity and excellent shifting characteristics, suitable for use as a transmission fluid for automatic transmissions and/or continuously variable transmissions, and such a lubricating oil composition is importantly enhanced in anti-shudder properties and suppress wet-type friction materials from wearing while retaining the foregoing properties. Further, there has been a demand for a transmission fluid having no problem in exhibiting the foregoing properties for carbon-containing friction materials, composed of mainly aramid fiber or cellulose fiber and containing fiber or filler made of carbon.                (1) Patent Document 1: Japanese Patent Laid-Open Publication No. 2003-113391        (2) Patent Document 2: Japanese Patent Laid-Open Publication No. 2001-323292        (3) Patent Document 3: Japanese Patent Laid-Open Publication No. 2000-355695        (4) Patent Document 4: Japanese Patent Laid-Open Publication No. 11-50077        (5) Patent Document 5: Japanese Patent Laid-Open Publication No. 10-306292        (6) Patent Document 6: Japanese Patent Laid-Open Publication No. 10-25487        (7) Patent Document 7: Japanese Patent Laid-Open Publication No. 2000-63869        